Conventional printers generally adapted to record images provided from digital data onto photosensitive media apply light exposure energy that may originate from a number of different sources and may be modulated in a number of different ways. In photoprocessing apparatus, for example, light exposure energy can be applied from a CRT printer. In a CRT printer, the digital data is used to modulate a cathode ray tube (CRT) which provides exposure energy by scanning an electron beam of variable intensity along its phosphorescent screen. Alternately, light exposure energy can be applied from a laser printer, as is disclosed in U.S. Pat. No. 4,728,965 (Kessler et al.) In a laser-based printer, the digital data is used to modulate the duration of laser on-time or intensity as the beam is scanned by a rotating polygon onto the imaging plane.
CRT and laser printers perform satisfactorily for photoprocessing applications, that is, for printing of photographs for consumer and commercial markets. However, in an effort to reduce cost and complexity, alternative technologies have been considered for use in photoprocessing printers. Among suitable candidate technologies under development are two-dimensional spatial light modulators.
Two-dimensional spatial light modulators, such as those using a digital micromirror device (DMD) from Texas Instruments, Dallas, Tex., or using a liquid crystal device (LCD) can be used to modulate an incoming optical beam for imaging. A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate incident light from a light source by, for instance, in the case of a LCD modulator, modulating the polarization state of the light. Polarization considerations are, therefore, important in the overall design of support optics for a spatial light modulator.
There are two basic types of spatial light modulators in current use. The first type developed was the transmissive spatial light modulator, which, as its name implies, operates by modulating an optical beam that is transmitted through individual array elements. The second type, a later development, is a reflective spatial light modulator. As its name implies, the reflective spatial light modulator operates by modulating a reflected optical beam through individual array elements. A suitable example of an LCD reflective spatial light modulator relevant to this application utilizes an integrated CMOS backplane, allowing a small footprint and improved uniformity characteristics.
Conventionally, LCD spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 (Konno et al.) and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800 (Handschy et al.) LCD projector and display designs in use typically employ one or more spatial light modulators, such as using one for each of the primary colors, as is disclosed in U.S. Pat. No. 5,743,610 (Yajima et al.).
It is instructive to note that imaging requirements for projector and display use (as is typified in U.S. Pat. Nos. 5,325,137; 5,808,800; and 5,743,610) differ significantly from imaging requirements for printing. Projectors are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for projector and display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as “forgiving” to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, projection and display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Photographic printing apparatus, on the other hand, must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems. Thus, while LCD spatial light modulators can be used in a range of imaging applications from projection and display to high-resolution printing, the requirements on supporting optics can vary significantly.
Largely because spatial light modulators can offer significant advantages in cost and size, these devices have been proposed for different printing systems, from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748 (Sarraf), to area printing systems such as the system described in U.S. Pat. No. 5,652,661 (Gallipeau et al.) One approach, using a Texas Instruments DMD as shown in U.S. Pat. No. 5,461,411 (Florence et al.) offers advantages common to spatial light modulator printing such as longer exposure times using light emitting diodes as a source as shown in U.S. Pat. No. 5,504,514 (Nelson). However, DMD technology is very specific and not widely available. As a result, DMDs may be expensive and not easily scaleable to higher resolution requirements. The currently available resolution using DMDs is not sufficient for all printing needs. Furthermore, there is no clear technology path to increased resolution with DMDs.
A preferred approach for photoprocessing printers uses an LCD spatial light modulator. Liquid crystal modulators can be a low cost solution for applications requiring spatial light modulators. Photographic printers using commonly available LCD technology are disclosed in U.S. Pat. Nos. 5,652,661; 5,701,185 (Reiss et al.); and U.S. Pat. No. 5,745,156 (Federico et al.) Although the present application primarily addresses use of LCD spatial light modulators, references to LCD in the subsequent description can be generalized, for the most part, to other types of spatial light modulators, such as the DMD noted above.
Primarily because of their early development for and association with screen projection of digital images, spatial light modulators have largely been adapted for continuous tone (contone) color imaging applications. Unlike other digital printing devices, such as the CRT and laser-based devices mentioned above that scan a beam in a two-dimensional pattern, spatial light modulators image one complete frame at a time. Using an LCD, the total exposure duration and overall exposure energy supplied for a frame can be varied as necessary in order to achieve the desired image density and to control media reciprocity characteristics. Advantageously, for photoprocessing applications, the capability for timing and intensity control of each individual pixel allows an LCD printer to provide grayscale imaging.
Most printer designs using LCD technology employ the LCD as a transmissive spatial light modulator, such as is disclosed in U.S. Pat. Nos. 5,652,661 and 5,701,185. However, the improved size and performance characteristics of reflective LCD arrays have made this technology a desirable alternative for conventional color photographic printing, as is disclosed in U.S. Pat. No. 6,215,547 (Ramanujan et al.) As is described in U.S. Pat. No. 6,215,547, color photographic printing requires multiple color light sources applied in sequential fashion. The supporting illumination optics are required to handle broadband light sources, including use of a broadband beamsplitter cube. The optics system for such a printer must provide telecentric illumination for color printing applications. In summary, in the evolution of photoprocessing systems for film printing, as outlined above, it can be seen that the contone imaging requirements for color imaging are suitably met by employing LCD spatial light modulators as a solution.
Printing systems for micrographics or computer output microfilm (COM) imaging, diagnostic imaging, and other specialized monochrome imaging applications present a number of unique challenges for optical systems. In the COM environment, images are archived for long-term storage and retrievability. Unlike conventional color photographic images, microfilm archives, for example, are intended to last for hundreds of years in some environments. This archival requirement has, in turn, driven a number of related requirements for image quality. For image reproduction quality, for example, one of the key expectations for micrographics applications is that all images stored on archival media will be written as high-contrast black and white images. Color film is not used as a medium for COM applications since it degrades much too quickly for archive purposes and is not capable of providing the needed resolution. Grayscale representation, meanwhile, has not been available for conventional micrographics printers. Certainly, bitonal representation is appropriate for storage of alphanumeric characters and for standard types of line drawings such as those used in engineering and utilities environments, for example. In order to record bitonal images onto photosensitive media, exposure energy applied by the printer is either on or off, to create high-contrast images without intermediate levels or grayscale representation.
In addition to the requirement for superb contrast is the requirement for high resolution of COM output. COM images, for example, are routinely printed onto media at reductions of 40× or more. Overall, micrographics media is designed to provide much higher resolution than conventional dye-based media provides for color photographic imaging. To provide high resolution, micrographics media employs a much smaller AgX grain size in its photosensitive emulsion. Optics components for COM systems are correspondingly designed to maximize resolution, more so than with optical components designed for conventional color photoprocessing apparatus.
Conventional COM printers have utilized both CRT and laser imaging optics with some success. However, there is room for improvement. For example, CRT printers for COM use, such as disclosed in U.S. Pat. No. 4,624,558 (Johnson) are relatively costly and can be bulky. Laser printers, such as disclosed in U.S. Pat. No. 4,777,514 (Theer et al.) present size and cost constraints and can be mechanically more complex, since the laser imaging system with its spinning polygon and beam-shaping optics must be designed specifically for the printer application. In addition, laser printers exhibit high-intensity reciprocity failure when used with conventional photosensitive media, thus necessitating the design of special media for COM use.
More recent technologies employed for COM imaging include use of linear arrays such as linear light-emitting diode (LED) arrays, for example, as are used in the Model 4800 Document Archive Writer, manufactured by Eastman Kodak Company, Rochester, N.Y. Another alternative is use of a linear light-valve array, such as is disclosed in U.S. Pat. No. 5,030,970 (Rau et al.) However, with exposure printheads using linear arrays, COM writers continue to be relatively expensive, largely due to the cost of support components and to the complexity of drive electronics. There is a long-felt need to lower cost and reduce size and complexity for COM devices, without sacrificing performance or robustness.
A well-known shortcoming of conventional COM printers relates to the use of microfilm for standard document page sizes. Conventionally, microfilm has been used for 11×14 inch computer output documents, for letter-sized documents (8.5×11 inches) or for A4 size documents (approximately 8.27×11.69 inches, 210×297 mm). Standard 16 mm microfilm allows documents having these sizes to be reduced by suitable factors, typically ranging from 20× to 50× reduction. Using different reduction ratios, documents can be arranged in different ways along the film. For conventional 16 mm film, there are standard simplex or “1-up” arrangements at lower reduction ratios and “2-up” arrangements at higher reduction ratios, with ratios often commonly agreed upon by COM equipment and media manufacturers. However, the use of 16 mm microfilm severely constrains the maximum size of documents that can be faithfully preserved in reduced form. For storage of larger documents, such as A2 size (16.54×23.39 in, 420×594 mm) or larger, 16 mm microfilm is unsatisfactory.
To store larger documents, a larger format microfilm, such as 35 mm microfilm, may be more appropriate. The larger 35 mm format allows high-quality digital printing of A2 and larger documents onto COM media at standard reduction ratios. For example, engineering drawings that have traditionally been archived using aperture cards may now be conveniently stored on 35 mm microfilm using digital COM film writers.
Relatively new for digital printing applications, the 35 mm film allows greater potential flexibility not only for storage of larger documents, but also where documents may need to be stored at lower reduction ratios. Some types of documents, for example, may have image content such as fine lines or highly detailed areas that cannot be faithfully preserved at 24:1 or greater reduction. Both for larger documents at high reduction ratios and for smaller documents, the 35 mm media also allows enhanced flexibility, allowing alternate arrangements of images on the COM media. For example, different arrangements could be proposed for storing color separations, such as red, green, and blue additive color separations or cyan, magenta, and yellow subtractive color separations, where the separations themselves are printed on COM media in monochromatic or grayscale form.
Some types of COM printing apparatus have been designed to print onto the larger 35 mm microfilm media and thereby provide the advantages that result from enhanced flexibility of image formats. As one example, the Microbox Polycom Laser Plotter manufactured by Microbox, located in Bad Nauheim, Germany is a COM imaging apparatus employing laser scanning, designed to use 35 mm format. However, conventional COM printing apparatus that are designed for imaging onto the larger-format 35 mm media do not provide efficient and affordable solutions for imaging onto the smaller-format 16 mm media. Using conventional COM imaging optics, the cost and complexity of a COM printing apparatus can be prohibitive. For example, when compared against optical requirements for 16 mm imaging, use of the larger 35 mm format requires proportionally larger beam incident angles in an apparatus using scanning techniques such as laser and CRT devices employ. Complex and expensive optical components are needed in order to suppress the effects of increased aberration. In rotating polygon systems, for example, motion-induced optical artifacts are substantially more pronounced when imaging in a larger 35 mm format. In the case of linear array printing methods, extending printhead length to suit the larger 35 mm format also requires considerably more cost and complexity than are needed for 16 mm imaging.
In addition to cost and complexity disadvantages of conventional 35 mm COM imaging apparatus, conventional COM imaging approaches make these apparatus inherently less efficient for smaller-format 16 mm imaging. There are no throughput benefits in imaging to a smaller-format COM media, since conventional scanning designs fix scan sequences, sweep angles, and timing to suit larger-format media. Likewise for linear array imaging devices, imaging onto a smaller-format media is less efficient, since only a portion of the available printhead optics can be used. The above-mentioned drawbacks of increased cost and complexity and reduced efficiency render conventional approaches unsatisfactory for variable-format COM imaging in a cost-sensitive and efficiency-driven market.
A further drawback of conventional COM imaging approaches relates to productivity constraints inherent to scanning and to line array imaging devices. Conventional COM imaging methods, which operate generally by exposing pixels in a line-by-line fashion, are not easily adapted to take advantage of expanded possibilities for using varied imaging formats and of opportunities for writing multiple images in a single exposure.
An attempt at designing an apparatus capable of addressing these needs was presented in U.S. Pat. No. 6,552,740 (Wong et al.) which depicts a monochromatic printer based on spatial light modulator technology. Prior art U.S. Pat. No. 6,580,490 (Wong et al.) also demonstrates the use of a liquid crystal on Silicon (LCOS) device for printing in multiple formats. U.S. Pat. No. 6,480,259 (Wong et al.) depicts a printer capable of selecting light sources. However, LCOS devices and the surrounding optical system are thermally sensitive. While the prior art printer can create images at an image plane, variations in temperature at the LCOS device can alter the image uniformity and quality. Knowledge of this effect can be utilized to improve the design by not only controlling the temperature gradients to prevent unwanted uniformity changes, but to correct uniformity errors already present in the printing system.
Thus, it can be seen that there is a need for an improved COM printing apparatus that is inexpensive, compact, and robust, and that allows printing in any of a plurality of output media formats, including printing of multiple images at one time while maintaining thermal control of the system.
An additional application within the field of printing is that of printing of x-ray images. X-ray images are printed on dry silver media as well as wet processed silver. These black and white images are created through infra-red exposure. Available printers for imaging the digital data on dry silver media include laser based printers. With the increase in digital capture, the need for low cost, high speed engines for writing on dry silver will increase. Use of spatial light modulators in such printing applications will help reduce cost. In additions, dry silver media is thermally processed. Consequently, care must be taken to provide a thermally controlled environment at the media.
Thus, it can be seen that there is a need for an improved printing apparatus that is inexpensive, compact, and robust, and that is thermally controlled for optimal use.